Method and apparatus for performing handover of user equipment in wireless communication system supporting dual connectivity

ABSTRACT

A method of supporting a Handover (HO) by a master evolved Node B (eNB) in a wireless communication system supporting dual connectivity of a User Equipment (UE) for the master eNB and a slave eNB is provided. The method includes when an HO of the UE to the slave eNB is determined, transmitting an HO request message to a target slave eNB, when a HO Ack message is received from the target slave eNB, transmitting an HO trigger request message including identification information of the target slave eNB to a source slave eNB, and when a resource release message is received from the target slave eNB, transmitting an HO success indicator to the source slave eNB.

PRIORITY

This application claims priority under 35 U.S.C. § 119(a) to KoreanPatent Application No. 10-2014-0011821, which was filed in the KoreanIntellectual Property Office on Jan. 29, 2014, the entire invention ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless communicationsystem, and more particularly, to a method and an apparatus forperforming a Handover (HO) of a User Equipment (UE) in a wirelesscommunication system supporting dual connectivity.

2. Description of the Related Art

In general, a mobile communication system was developed to provide voiceservices, and has gradually expanded its service area to include a dataservice as well as a voice service and has recently been developed toprovide a high speed data service. However, since resources are lackingand users are demanding higher speed services from mobile communicationsystems currently providing services, an improved mobile communicationsystem is needed.

To meet the demand for wireless data traffic having increased sincedeployment of 4G communication systems, efforts have been made todevelop an improved 5G or pre-5G communication system. Therefore, the 5Gor pre-5G communication system is also called a ‘Beyond 4G Network’ or a‘Post LTE System’.

The 5G communication system is considered to be implemented in higherfrequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higherdata rates. To decrease propagation loss of the radio waves and increasethe transmission distance, the beamforming, massive multiple-inputmultiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna,an analog beam forming, large scale antenna techniques are discussed in5G communication systems.

In addition, in 5G communication systems, development for system networkimprovement is under way based on advanced small cells, cloud RadioAccess Networks (RANs), ultra-dense networks, device-to-device (D2D)communication, wireless backhaul, moving networks, cooperativecommunication, Coordinated Multi-Points (CoMP), reception-endinterference cancellation and the like.

In the 5G system, Hybrid FSK and QAM Modulation (FOAM) and slidingwindow superposition coding (SWSC), as an advanced coding modulation(ACM), and filter bank multi carrier (FBMC), non-orthogonal multipleaccess (NOMA), and sparse code multiple access (SCMA), as an advancedaccess technology, have been developed.

Meanwhile, dual connectivity may mean that one UE is connected to twoEvolved Node Bs (eNBs) to receive services. For example, the dualconnectivity may mean that one UE is connected to as macro eNB and asmall (pico) eNB which have different functions to receive services.

A dual connectivity technology is being actively discussed by currentcommunication standard organizations. Particularly, in the dualconnectivity technology, a method of handing over a UE is urgentlyneeded since a procedure thereof has not yet been defined.

SUMMARY OF THE INVENTION

The present invention has been made to address at least the abovementioned problems and/or disadvantages and to provide at least theadvantages described below.

An aspect of the present invention provides a method of supporting an HOby a master eNB in a wireless communication system supporting dualconnectivity of a UE for the master eNB and a slave eNB. The methodincludes when an HO of the UE to the slave eNB is determined,transmitting an HO request message to a target slave eNB, when a HO Ackmessage is received from the target slave eNB, transmitting an HOtrigger request message including identification information of thetarget slave eNB to a source slave eNB and when a resource releasemessage is received from the target slave eNB transmitting an HO successindicator to the source slave eNB.

An aspect of the present invention provides a method of supporting an HOby a master eNB in a wireless communication system supporting dualconnectivity of a UE for the master eNB and a slave eNB. The methodincludes when an HO of the UE to the master eNB is determined,transmitting an HO request message to a target master eNB, when an HOAck message is received from the target master eNB, transmitting an HOtrigger indicator to a source slave eNB, and when a resource releasemessage is received from the target master eNB, transmitting an HOsuccess indicator to the source slave eNB.

Another aspect of the present invention provides a master eNB forsupporting an HO in a wireless communication system supporting dualconnectivity of a UE for the master eNB and a slave eNB. The master eNBincludes a transceiver configured to transmit/receive signals to/fromthe UE or nodes of the wireless communication system and a controllerconfigured to transmit an HO request message to a target slave eNB whenan HO of the UE to the slave eNB is determined, to transmit an HOtrigger request message including identification information for thetarget slave eNB to a source slave eNB when an HO Ack message isreceived from the target slave eNB, and to transmit an HO successindicator to the source slave eNB when a resource release message isreceived from the target slave eNB.

Another aspect of the present invention provides a master eNB forsupporting an HO in a wireless communication system supporting dualconnectivity of a UE for the master eNB and a slave eNB. The master eNBincludes a transceiver configured to transmit/receive signals to/fromthe UE or nodes of the wireless communication system and a controllerconfigured to transmit an HO request message to a target master eNB whenan HO of the UE to the master eNB is determined, to transmit an HOtrigger indicator to a source slave eNB when an HO Ack message isreceived from the target master eNB, and to transmit an HO successindicator to the source slave eNB when a resource release message isreceived from the target master eNB.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptionin conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating characteristics of dualconnectivity, according to an embodiment of the present invention;

FIG. 2 is a signaling diagram illustrating an HO process in a singleconnectivity system, according to an embodiment of the presentinvention;

FIGS. 3A and 3B are diagrams illustrating various types of dualconnectivity, according to an embodiment of the present invention;

FIG. 4 is a signaling diagram illustrating a pico link (HO) process of aRAN split scenario, according to an embodiment of the present invention;

FIG. 5 is a signaling diagram illustrating a macro link HO process of aRAN (Radio a Access Network) split scenario, according to an embodimentof the present invention;

FIG. 6 is a signaling illustrating a pico link HO process of a CN (CoreNetwork) split scenario, according to an embodiment of the presentinvention;

FIG. 7 is a signaling diagram illustrating a detailed operation ofsequences of a macro link HO process of a CN split scenario, accordingto an embodiment of the present invention;

FIG. 8 is a signaling diagram it minting an HO process as it pertains toa pico HO first start and a pico HO first end in a RAN split, accordingto an embodiment of the present invention;

FIG. 9 is a signaling diagram illustrating an HO process as it pertainsto a pico HO first start and a macro HO first end in a RAN split,according to an embodiment of the present invention;

FIG. 10 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a macro HO first end in a RAN split,according to an embodiment of the present invention;

FIG. 11 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a pico HO first end in a RAN split,according to an embodiment of the present invention;

FIG. 12 is a signaling diagram illustrating an HO process as it pertainsto a pico HO first start and a pico HO first end in a CN split,according to an embodiment of the present invention;

FIG. 13 is a signaling diagram illustrating an HO process as it pertainsto a pico HO first start and a macro HO first end in a CM split,according to an embodiment of the present invention;

FIG. 14 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a macro HO first end in a CN split,according to an embodiment of the present invention;

FIG. 15 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a pico HO first end in a CN split,according to an embodiment of the present invention;

FIG. 16 is a signaling diagram illustrating an HO process, according toanother embodiment of the present invention; and

FIG. 17 is a block diagram illustrating an internal structure of an eNBwhich can be applied to various eNBs, according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Hereinafter, embodiments of the present invention are described indetail with reference to the accompanying drawings. Those of ordinaryskill in the art will recognize that various changes and modificationsof the embodiments described herein can be made without departing fromthe scope of the present invention. In addition, descriptions ofwell-known functions and constructions may be omitted for clarity andconciseness. The same reference symbols are used throughout the drawingsto refer to the same or like parts.

It should be noted that various embodiments described below may beapplied or used individually or in combination.

FIG. 1 is a block diagram illustrating characteristics of dualconnectivity, according to an embodiment of the present invention.

A UE 100 is simultaneously connected to both a macro eNB 110 and a picoeNB 120. The macro eNB 110 and the pico eNB 120 provide services to theUE 100.

A radio resource control function may be provided to the UE 100 by boththe macro eNB 110 and the pico eNB 120 or by one of the macro eNB 110and the pico eNB 120.

A main objective of the dual connectivity is to increase throughput ofthe UE 100 through connections between the UE 100 and one or more eNBsfor data transmission/reception. Further, one eNB which is the macro eNB110 may serve as an anchor point for mobility by serving as a Master eNB(MeNB) to help with processing mobility of the UE 100.

FIG. 2 is a signaling diagram illustrating an HO process in a singleconnectivity system, according to an embodiment of the presentinvention.

As illustrated in FIG. 2, an HO preparation step may start after asource eNB 210 determines an HO (by selecting a target eNB) based on ameasurement report from the UE 200.

The source eNB 210 determines whether to accept an HO request andtransmits an HO request message to a selected target eNB 220. When thetarget eNB 220 accepts the UE 200 based on Quality of Service (QoS) ofan active flow of the UE 200 and a load condition thereof, an HOexecution step is initiated. Otherwise, the source eNB 210 may notselect another eNB as the target eNB.

In the HO execution step, the source eNB 210 transmits buffers andin-transit packets to the target eNB 220 and transmits an HO command tothe UE 200.

The UE 200 having received the HO command starts wireless interfacesynchronization with the target eNB 220, completes the synchronization,and then transmits an HO completion to the target eNB 220. After thetarget eNB 220 receives the HO completion, an HO completion step isinitiated in which the target eNB 220 starts a path switch process witha source gateway 230.

After the path, switch is performed, the target eNB 220 informs thesource eNB 210 that the HO has been sufficiently completed.

Thereafter, the source eNB 210 releases resources of the UE 200.

The above description will be explained in more detail with respect toFIG. 2. At step 0, an area restriction is provided. At step 1,measurement control is initiated at the UE 200. At step 2, measurementreports are exchanged between the UE 200 and the source eNB 210. At step3, an HO decision is determined at the source eNB 210. At step 4, an HOrequest is transmitted from the source eNB 210 to the target eNB 220. Atstep 5, an admission control is initiated. At step 6, an HO requestAcknowledgement (Ack) is transmitted from the target eNB 220 to thesource eNB 210. At step 7, an HO command signal is transmitted from thesource eNB 210 to the UE 200. At step 8, an SN (Sequence Number) statustransfer signal is transferred from the source eNB 210 to the target eNB220. At step 9, synchronization between the UE 200, the source eNB 210,and the target eNB 220 is performed. At step 10, UL allocation and TAallocation is provided for UE 200. At step 11, an HO complete signal istransmitted from the UE 200 to the target eNB 220. At step 12, a path,switch request signal is transmitted from the target WE 220 to an MME(Mobility Management Entity) 240. At step 13, a modify bearer requestsignal is transmitted from the MME 240 to the source gateway 230. Atstep 14, a switch DL path signal is transmitted from the source gateway230 to the source eNB 210. At step 15, a modify bearer response signalis transmitted from the source gateway 230 to the MME 240. At step 16, apath switch request Ack signal is transmitted from the MME 240 to thetarget eNB 220. At step 17, a release resource signal is transmittedfrom the target eNB 220 to the source eNB 210. At step 18, the sourceeNB 210 determines to release resources.

FIGS. 3A and 3B are diagrams illustrating various types of dualconnectivity, according to an embodiment of the present invention.

First, FIG. 3A illustrates a structure in which a bearer formed betweena data network and a UE is split from a core network.

As illustrated in FIG. 3A, when a first EPS (Evolved Packet Core) bearerand a second EPS bearer are formed between the data network and the UE,the first EPS bearer is connected to the UE via an MeNB in the corenetwork, but the second EPS bearer is split from the core network node(for example, a Source Gateway (SGW)) into an SeNB and connected to theUE.

FIG. 3B illustrates a structure in which a bearer formed between a datanetwork and the UE is split from a radio access network.

As illustrated in FIG. 3B, when a first EPS bearer and a second EPSbearer are formed between the data network and the UE, the first EPSbearer is connected to the UE via a MeNB in the core network, but thesecond EPS bearer is split from an RAN node, for example, the MeNB intoan SeNB and connected to the UE.

With continued reference to FIGS. 3A and 3B, a process of handing overthe MeNB and the Selma according to the types of dual connectivityillustrated in FIGS. 3A and 3B is now described.

Further, the HO according to the present invention may be classifiedinto a HO for a single link and a HO for a dual link.

The HO for the single link may mean that, in a state where the UE isconnected to a macro eNB and a pico eNB a link with the macro eNB or alink with the pico eNB is handed over.

The HO for the dual link may mean that both the link with the macro eNB(and the like) and the pico eNB are handed over.

The MeNB may refer to a macro eNB and the SeNB may refer to a small eNBor a pico eNB. More specifically, the macro eNB may be the MeNB leadingthe HO, and the small eNB or the pico eNB may be the SeNB performing theHO, according to an instruction from the MeNB.

Further, although it is assumed that the macro eNB is the MeNB and thesmall eNB or the pico eNB is the SeNB in the following embodiments, thepresent invention is not limited thereto and vice versa.

First, a HO method for the single link will be described with referenceto FIGS. 4-7.

Combinations of the HO for the single link and the types of dualconnectivity may be specifically divided into a pico link HO process ofan RAN split scenario, a macro link HO process or an RAN split scenario,a pico link HO process of a CN split scenario, and a macro link HOprocess of a CN split scenario.

The pico link HO process of the RAN split scenario will be describedfirst.

In the RAN split scenario, when a HO of the pico eNB in dualconnectivity is performed by the macro eNB after an HO is determined,the macro eNB transmits an HO request to a selected target pico eNB, andthe pico eNB performs an admission control based on the HO request andthen transmits an Ack to the macro eNB.

When receiving a positive Ack, the macro eNB transmits the request to asource pico eNB to trigger data and context transmission to the targetpico eNB, and also transmits an HO command to the UE.

When receiving the HO command, the UE starts radio interfacesynchronization with the target pico eNB, completes the synchronization,and then transmits an HO completion to the target pico eNB.

When receiving the HO completion, the target pico eNB transmits an HOAck indication to the macro eNB. The macro eNB switches thecorresponding flow to the pico eNB and transmits the indication to thesource pico eNB, so as to release resources for the UE.

Data transmission is performed by the target pico eNB together with themacro eNB for providing the UE in a dual connectivity state.

The above process will be described in detail with reference to FIG. 4.

FIG. 4 is a signaling diagram illustrating a pico link HO process of aRAN split scenario, according to an embodiment of the present invention.

As illustrated in FIG. 4, at step S405, a UE 400 triggers an HO on anSeNB link. Accordingly, the UE 400 transmits a measurement reportmessage to a MeNB 410, at step to S410, and the MeNB 410 determineswhether to perform the HO based on the measurement report, at step S415.

Further, the MeNB 410 transmits an HO request message to a target picoeNB 430, at step S420. The HO request message may include indicationinformation indicating that the HE 400 is in a dual connectivity mode,information indicating that the HO is from a source SeNB ID, and flowinformation related to the source SeNB.

The target pico eNB 430 does not perform a HO process with aconventional core network, at step S425. That is, the Ho described inFIG. 4 is for the RAN split. More particularly, since the target picoeNB 430 is connected to the MeNB 410, the target pico eNB 430 does notneed to form a separate connection with the core network. Since thetarget pico eNB 430 is connected to a core network node (for example, anSGW) in the CN split, which will be described below, the target pico eNB430 needs to perform the HO process with the core network.

Further, the target pico eNB 430 transmits an HO Ack message to the MeNB410, at step S430. Then, the MeNB 410 transmits a HO trigger requestmessage (pico) to a source pico eNB 420, at step S435. In this case, theHO trigger request message may include an identifier of the target picoeNB 430. The HO trigger request message may trigger the source SeNB 420to transmit context information to the target eNB 430, which will bedescribed below.

Thereafter, the MeNB 410 buffers a data packet which has beentransmitted to the source pico SeNB 420.

Further, the source pico eNB 420 transmits context and data to thetarget pico eNB 430, at steps S450 and S460.

Thereafter, the UE 400 and the target pico eNB 430 performsynchronization, at step S465, and the UE 400 transmits a HO completionmessage to the target pico eNB 430, at step S470.

Then, the target pico eNB 430 transmits a resource release (HO Ack)message to the MeNB 410, at step S475. Then, the MeNB 410 switches flowsfor the UE 400 to the target pico eNB 430 from the existing source picoeNB 420, at step S480.

Further, the MeNB 480 transmits a HO success indicator (pico) to thesource pico eNB 420, at step S485, and the source pico eNB 420 deletescontext for the UE 400, at step S490.

Thereafter, the UE 400 transmits/receives data to/from the target picoeNB 430.

The above described embodiment will be summarized as follows.

HO Preparation Phase

-   -   HO request

Source macro→target pico

-   -   Indicate HO from pico eNB    -   Target pico eNB does not initiate any HO process in completion        phase

HO Execution Phase

-   -   HO trigger request (REQ)

Source macro→source pico

-   -   Source pico transmits only data and context    -   Source macro buffers only data packet of pico flow

HO Completion Phase

-   -   HO Ack

Target pico→source macro

Switch pico flows to target pico

-   -   HO success indicator (IND)

Source macro→source pico

Source pico deletes UE context

Next, the macro link HO process of the RAN split scenario will bedescribed.

In the RAN split scenario, when the HO of the macro eNB in a dualconnectivity state is performed after the HO is determined, the macroeNB transmits the HO request to the selected target macro eNB.

Then, the target macro eNB performs an admission control based on the HOrequest and transmits an Ack to the macro eNB. In the HO request, themacro eNB includes information indicating that the UE is in a dualconnectivity state with an eNB ID of the related pico eNB and flowinformation on flows related to the pico eNB.

When receiving a positive Ack, the macro eNB transmits an indication tothe related pico eNB to indicate the HO of the macro layer starts with amacro eNB ID, and also starts buffering of all flows and transmits an HOcommand to the UE. When receiving the indication, the pico eNB maybuffer any control message until the HO of the macro layer is completed.

When receiving the HO command, the UE starts radio interfacesynchronization with the target macro eNB, completes thesynchronization, and then transmits an HO completion to the target macroeNB. When receiving the HO completion, the target macro eNB performsconventional HO execution and completion phase processes.

After receiving the HO Ack from the target macro eNB, the source macroeNB transmits an indication to the related pico eNB to indicate that theHO is completed by the macro layer. Data transmission is performed bythe target macro eNB together with the related pico eNB to provide theUB in the dual connectivity state.

The above description will be made in detail with reference to FIG. 5.

FIG. 5 is a signaling diagram illustrating a macro link HO process of aRAN split scenario according to an embodiment of the present invention.

As illustrated in FIG. 5, at step S505, a UE 500 triggers a HO on a MeNBlink. Accordingly, the UE 500 transmits a measurement report message toa source macro eNB 510, at step S510, and the source macro eNB 510determines whether to perform the HO based on the measurement report, atstep S520.

Further, the source macro eNB 510 transmits a HO request message to atarget macro eNB 530, at step S520. The HO request message may includeindication information indicating that the UE is in a dual connectivitymode, an identifier of a source pico eNB in a dual connectivity mode,flow information related to the source pico eNB, and source macro eNBflow information.

Then, the target macro eNB 530 transmits a HO Ack message to the sourcemacro eNB 510, at step S540. Then, the source macro eNB 510 transmits aHO trigger indication message (macro) to a source pico eNB 520, at stepS555. In this case, the HO trigger indication message may include anidentifier of the target macro eNB 530. The HO trigger indicationmessage may instruct the source pico eNB 520 to buffer a dualconnectivity (DC) message (or a DC-related message) and then transmitthe DC message to the target macro eNB 530 at a proper time.

Further, the source pico eNB 520 may buffer the DC message in the sourcemacro eNB 510 until the HO is completed. The DC message may include oneor more pieces of information required for processing dual connectivity.The buffered DC message may be used for forming the dual connectivitybetween the source pico eNB 520 and the target macro eNB 530, at stepS594, which will be described below.

Thereafter, the source macro eNB 510 transmits a HO command message tothe UE 500, at step S565. Further, the source macro eNB 510 initiatesdata buffering for SeNB and MeNB flows, at step S570.

In addition, the source macro eNB 510 and the target macro eNB 530perform a HO execution process, at step S575.

The UE 500 transmits a HO completion message to the target macro eNB530, at step S580. Then, the source macro eNB 510 and the target macroeNB 530 perform a HO completion process, at step S585.

The target macro eNB 530 transmits a resource release message to thesource macro eNB 510, at step S590. Then, the source macro eNB 510transmits a HO success indicator to the source pico eNB 520, at stepS592, and dual connectivity is formed between the source pico eNB 520and the target macro eNB 530, at step S594. In this case, the DC messagethat is buffered at step S560 may be used.

The UE 500 transmits/receives data to/from the source pico eNB 520 andthe target macro eNB 530, at steps S596 and S598.

The above described embodiment may be summarized as follows.

HO Preparation Phase

-   -   HO request    -   Indicate that UE is in DC state with source pico (ID)    -   Progress flow information on bath macro and pico

HO Trigger IND

Source macro→source pico

-   -   source pico buffers DC message

HO Execution Phase

-   -   Apply conventional HO process

HO completion Phase

-   -   Apply conventional HO process    -   HO success IND

Source macro→source pico

Source pico performs DC with target macro

HO Request

-   -   Indicate UE DC mode (together with source pico ID)

Target macro should accept DC mode

-   -   Progress flow information on both macro and pico

HO Trigger IND

-   -   Target macro ID    -   Source pico buffers DC message until HO completion    -   UE ID

HO Success IND

-   -   Macro Layer

Source pico performs DC with target macro

-   -   UE ID

Next, the pico link HO process of the CN split scenario will bedescribed.

In the CN split scenario, when the pico eNB HO in the dual connectivitymode is performed after the HO is determined by the macro eNB, the macroeNB transmits an HO request including that the UE is in a dualconnectivity state together with the macro eNB to the selected targetpico eNB. The target pico eNB performs an admission control based on theHO request and transmits an Ack to the macro eNB.

When receiving a position Ack, the macro eNB transmits a request fortriggering data and context transmission to the target pico eNB to thesource pico eNB, and also transmits an HO command.

When receiving the HO command, the UE starts radio interfacesynchronization with the target pico eNB, completes the synchronization,and then transmits an HO completion to the target pico eNB.

When receiving the HO completion, the target pico eNB performs aconventional HO completion phase process and transmits an HO Ackindication to the macro eNB.

The macro eNB switches the corresponding flow to the pico eNB andtransmits an indication to the source pico eNB, so as to releaseresources for the UE. Data transmission is performed by the target picoeNB together with the macro eNB providing the UE in the dualconnectivity state.

A detailed process of the above phases will be described with referenceto FIG. 6.

FIG. 6 is a signaling illustrating a pico link HO process of a CN splitscenario, according to an embodiment of the present invention.

As illustrated in FIG. 6, at step S605, a UE 600 triggers an HO on anSeNB link. Accordingly, the UE 600 transmits a measurement reportmessage to an MeNB 610, at step S610, and the MeNB 610 determineswhether to perform the HO based on the measurement report, at step S615.

Further, the MeNB 610 transmits an HO request message to a target picoeNB 630, at step S620. The HO request message may include indicationinformation indicating that the HE 600 is in a dual connectivity mode,information indicating that the HO is from a source SeNB ID, and flawinformation related to the source SeNB.

Then, the target pico eNB 630 may perform a conventional HO process witha core network, at step S625. Further, the target pico eNB 630 transmitsan HO Ack message to the MeNB 610, at step S630.

Then, the MeNB 610 transmits an HO trigger request message (pico) to thesource pico eNB 620, at step S635. In this case, the HO trigger requestmessage may include an to identifier of the target pico eNB 530.Further, the MeNB 610 may transmit an HO command message to the HE 600,at step S640.

Further, the source pico eNB 620 buffers data an flows, at step S645,and triggers UE 600 context and data transmission, at step S650.Accordingly, the source pico eNB 620 transmits UE 600 context to thetarget pico eNB 630, at step S655, and transmits data to the target picoeNB 630, at step S660.

Thereafter, at step S665, the UE 600 performs synchronization with thetarget pico eNB 630. Further, the UE 600 transmits an HO completionmessage to the target pico eNB 630, at step S670.

Then, the source pico eNB 620 and the target pico SeNB 630 perform apath switch process, at step 675. A conventional path switch process maybe used as the path switch process.

The target pico eNB 630 may transmit a resource release message (HO Ack)to the MeNB 610, at step 680.

Then, the MeNB 610 transmits an HO success indicator (pico) to thesource pico eNB 620, at step S685, and the source pico eNB 620 deletesUE 600 context, at step S690.

Thereafter, the UE 600 and the target pico eNB 630 transmit/receivedata, at step S695.

The above described embodiment may be summarized as follows.

HO Preparation Phase

-   -   HO request

Source macro→target pico

Indicates HO from pico eNB

-   -   Start conventional HO process in target pico eNB completion step

HO Execution Phase

-   -   HO trigger REQ

Source macro→source pico

-   -   Source pico buffers data packets of flow    -   Source pico performs data and context transmission

HO Completion Phase

-   -   HO Ack

Target pico→source macro

-   -   After conventional path switch process is completed in core        network

HO Success IND

Source macro→source pico

Source pico deletes UE context

HO Request

-   -   Indicate UE DC mode

Target pico should accept DC mode

-   -   HO is from related source pico (together with ID)    -   Target pico accepts data transmitted from source pico

Information on now of only source pico

-   -   Admission control

HO Trigger REQ

-   -   Target pico ID

Transmit data to target pico

-   -   UE ID

HO Success IND

-   -   Pico layer    -   Delete UE context    -   UE ID

Next, the macro link HO process of the CN split scenario will bedescribed.

In the CN split scenario, when the HO of the macro eNB in a dualconnectivity state is performed after the HO is determined by the macroeNB, the macro eNB transmits the HO is request to the selected targetmacro eNB. The target macro eNB performs an admission control based onthe HO request and transmits an Ack to the macro eNB.

In the HO request, the macro eNB includes information indicating thatthe UE is in the ducal connectivity state together with an eNB ID of therelated pico eNB. When receiving a positive Ack, the macro eNB transmitsan indication indicating that an HO of a macro layer starts with thetarget macro eNB ID to the related pico eNB and also transmits an HOcommand.

When receiving the indication, the pico eNB may also buffer a controlmessage until the HO of the macro layer is completed.

When receiving the HO command, the UE starts radio interfacesynchronization with the target macro eNB, completes thesynchronization, and then transmits an HO completion to the target macroeNB.

When receiving the HO completion, the target macro eNB performsconventional HO execution and completion phase processes.

After receiving the HO Ack from the target macro eNB, the source macroeNB transmits an indication indicating that the HO is completed by themacro layer to the related pico eNB. Data transmission is performed bythe target macro eNB together with the related pico eNB providing the UEis the dual connectivity state.

The above process will be described in detail with reference to FIG. 7.

FIG. 7 is a signaling diagram illustrating a detailed operation ofsequences of a macro link HO process of a CN split scenario, accordingto an embodiment of the present invention.

As illustrated in FIG. 7, at step S705, a UE 700 triggers an HO on aMeNB link. Accordingly, the UE 700 transmits a measurement reportmessage to a source macro eNB 710, at step S710, and the source macroeNB 710 determines whether to perform the HO based on the measurementreport, at step S715.

Further, the source macro eNB 710 transmits an HO request message to atarget macro eNB 730, at step S720. The HO request message may includeindication information indicating that the UE 700 is in a dualconnectivity mode with a source SeNB ID and source MeNB flowinformation.

Then, the target macro eNB 730 transmits an HO Ack message to the sourcemacro eNB 710, at step S725.

Then, the source macro eNB 710 transmits an HO trigger indicator (macro)to a source pies) eNB 720, at step S730. In this case, the HO triggerindicator may include an identifier of the target macro eNB 730. Then,the source pico eNB 720 may buffer a DC message to the source macro eNB710 until the HO is completed, at step S735.

Further, the source macro eNB 710 transmits an HO command message to theUE 700, at step S740.

In addition, the source macro eNB 710 and the target macro eNB 730 mayperform an HO execution process, at step S745. The HO execution processmay follow the conventional HO execution process.

Further, the UE 700 may transmit an HO completion message to the targetmacro eNB 730, at step S750. Then, the source macro eNB 710 and thetarget macro eNB 730 may perform an HO completion process, at step S755.The HO completion process may follow the conventional HO completionprocess.

The target macro eNB 730 may transmit resource release (HO Ack) messageto the source macro eNB 710, at step S760. Then, the source macro eNB710 may transmit an HO success indicator to the source pico eNB 720, atstep S765.

At step S770, dual connectivity is formed between the source pico eNB720 and the is target macro eNB 730.

The UE 700 and the target macro eNB 730 may perform datatransmission/reception, at step S775.

The above described embodiment may be summarized as follows.

HO Preparation Phase

-   -   HO request    -   Indicate that UE is in DC state with source pico (ID)    -   HO trigger IND

Source macro→source pico

-   -   Source pico buffers DC message

HO Execution Phase

-   -   Apply conventional HO process    -   HO completion phase    -   Apply conventional HO process

HO Success IND

-   -   Source macro→source pico    -   Source pico performs DC with target macro

HO Request

-   -   Indicate UE DC mode (together with source pico ID)    -   Target macro should accept DC mode

HO Trigger IND

-   -   Target macro ID    -   Source pico buffers DC message until HO completion    -   UE ID

HO Success IND

-   -   Macro layer    -   Source pico performs DC with target macro    -   UE ID

Meanwhile, the embodiments disclosed in FIGS. 4-7, that is, the maincharacteristics of the single link HO are summarized as follows.

HO Request

-   -   information content    -   Indicate UE DC mode together with ID of related eNB    -   Flows of related eNB (except flows thereof)        -   RAN split    -   Use    -   Macro HO    -   Source macro→target macro    -   Pico HO

Source macro→target pico

HO Trigger IND

-   -   Information content    -   Layer of HO    -   Target ONE ID    -   UE ID    -   Use    -   Macro HO    -   Source macro→source pico    -   Inform of macro HO

HO Trigger REQ

-   -   Information content    -   UE ID    -   Use    -   Pico HO    -   Source macro→source pico        -   Triggers pico HO            -   In different way from conventional way

HO Success END

-   -   Information content    -   Layer on which HO is completed    -   UE ID    -   Use

Source macro→source pico

-   -   Inform of HO completion

Hereinafter, detailed embodiments of the dual link HO will be described.

Main characteristics of a dual link HO process will be first describedbelow. The dual link HO starts from one layer as a single link HOprocess. In this case, the dual link HO is one part of the single linkHO process, and the HO starting from one layer is displayed in anotherlayer.

Further, completion of the HO of any one layer is displayed in anotherlayer of which the HO is ongoing. In this case, the above matter isrequired to be notified to both source and target entities of anotherlayer. The source entity should be informed through the part of thesingle link HO process. The target entity needs to be informedadditionally.

Further, completion of the HO of another layer may be notified to alayer of which the HO has been completed. This may mean that thecompletion of the HO is notified to an active (target) node.

When the HO of a macro layer is first completed, the source macro maycontinuously control an HO process between the eNBs until the HO of apico layer is completed.

As another characteristic, a difference between the CN split and the RANsplit will be described below. In the CN split, after the HO on the picolayer is completed, flows on the pico layer may temporarily stop.Conversely, in the RAN split, after the HO on the pico layer and the HOon the macro layer are completed, flows on the pico layer maytemporarily stop.

When the HO on the pico layer is being completed and the HO on the macrolayer is ongoing, the source macro may process a reconfiguration of thetarget pico.

Hereinafter, a detailed HO process according to the type of dual link HOwill be described.

First, a pico HO first start and a pico HO first end in the RAN splitwill be described.

According to the embodiments of the present invention, the HO of themacro eNB starts in the RAN split scenario in which the HO of the picoeNB has started, but has not yet been completed.

The macro eNB, having determined the HO, transmits an HO request, whichincludes information indicating that the UE is in the dual connectivitystate with a source pico eNB ID, information indicating that the picoeNB HO is ongoing with the target pico eNB ID, and information on allflows, to the selected target eNB.

The target macro eNB performs an admission control and transmits an Ackto the source macro eNB. The source macro eNB transmits the indicationindicating that the HO on the macro layer is ongoing with the targetmacro eNB ID to both the source and target pico eNBs, and transmits anHO command to the UE.

When the HO on the macro layer is ongoing and the HO on the pico layerhas been completed, the target pico eNB transmits the HO Act to thesource macro eNB. The source macro eNB informs the source pico eNB andthe target macro eNB of the HO completion on the pico layer. Datatransmission may be performed by the source macro eNB together with thetarget pico eNB. When the HO on the macro layer is completed, the targetmacro eNB transmits, to the source macro eNB, the HO Ack that informsthe target pico eNB (active eNB on the pico layer) of the completion ofthe HO on the macro layer. Data transmission may be performed by thetarget macro eNB together with the target pico eNB.

The above process will be described in detail with reference to FIG. 8.

FIG. 8 is a signaling diagram illustrating a HO process as it pertainsto a pico HO first start and a pico HO first end in a RAN split,according to an embodiment of the present invention.

First, a UE 800 detects triggering of an HO event on a pico link, atstep S805. Then, the UE 800 transmits a measurement report to an MeNB810, at step S810. The UE 800 and a target pico eNB 830 perform the sameprocess as that of the RAN split pico HO, at step 815.

Thereafter, the UE 800 detects triggering of an HO event on the macrolink, at step S820. Then, the UE 800 transmits the measurement report tothe MeNB 810, at step S825. The MeNB 810 transmits an HO request messageto a target macro eNB 830, at step S830. The HO request message mayinclude information indicating that the UE 800 is in the dualconnectivity state, an identifier of the source SeNB for dualconnectivity, information indicating that the SeNB HO is ongoing, atarget SeNB identifier and source MeNB and SeNB flow information.

Thereafter, the target macro eNB 830 may transmit an HO Ack message tothe MeNB 810, at step S835.

Then, the MeNB 810 may transmit an HO trigger indicator (macro) to thesource pico eNB 820, at step S840. The HO indicator may include theidentifier of the target macro eNB 830.

Further, the MeNB 810 may transmit the HO trigger indicator (macro) tothe target pico eNB 840, at step S845. The HO trigger indicator mayinclude the identifier of the target macro eNB 830.

Then, the target pico eNB 840 completes the HO process, at step S850.The target pico eNB 840 transmits a resource release (HO Ack) message tothe MeNB 810, at step S855.

Then, the MeNB 810 may transmit an HO success indicator (pico) to thesource pico eNB 820, at step S860. Further, the MeNB 810 transmits theHO success indicator (pico) to the target macro eNB 840830, at stepS865.

Then, the target macro eNB 830 completes the HO process, at step S870,and transmits a resource release (HO Ack) message to the MeNB 810, atstep S875.

The MeNB 810 transmits an HO success indicator (macro) to the targetpico eNB 840, at step S880. The HO success indicator may includeidentifier information on the target macro eNB 830.

Thereafter, the UE 800 and the target pico eNB 840 transmit/receivedata, at step S885, and the UE 800 and the target macro eNB 830transmit/receive data, at step S890.

Next, a pico HO lint start and a macro HO first end in the RAN splitwill be described.

In the above described embodiment of the present invention, after themacro eNB HO starts, the macro eNB, having determined the HO transmitsan HO request, which includes information indicating that the UE is inthe dual connectivity state with a source pico eNB ID, informationindicating that the pico eNB HO is ongoing with the target pico eNB IDand, information on all flows, to the selected target eNB.

The target macro eNB performs an admission control and transmits Ack tothe source macro eNB.

The source macro eNB transmits the indication indicating that the HO onthe macro layer is ongoing with the target macro eNB ID to both thesource and target pico eNBs, and transmits an HO command to the UE.

When the HO on the pico layer is ongoing and the HO on the macro layerhas been completed, the target macro eNB transmits the HO Ack to thesource macro eNB. The source macro eNB informs the source pico eNB andthe target pico eNB of the HO completion on the macro layer.

Data transmission may be performed by the target macro eNB together withthe source pico eNB.

When the HO on the pico layer is completed, the target pico eNBtransmits, to the source macro eNB, the HO Ack that informs the targetmacro eNB and the source pico eNB of the completion of the HO on thepico layer. Data transmission may be performed by the target macro eNBtogether with the target pico eNB.

The above process will be described in detail with reference to FIG. 9.

FIG. 9 is a signaling diagram illustrating an HO process as it pertainsto a pico HO first start and a macro HO first end in a RAN split,according to an embodiment of the present invention.

A UE 900 first detects triggering of an HO event on the pico link, atstep S905. Then, the UE 900 transmits a measurement report to an MeNB910, at step S910. The UE 900 and a target pico eNB 930 perform the sameprocess as that of the RAN split pico HO.

Thereafter, the UE 900 detects triggering of an HO event on the macrolink, at step S920. Then, the UE 900 transmits a measurement report tothe MeNB 910, at step S925. The MeNB 910 transmits an HO request messageto a target macro eNB 940, at step S930. The HO request message mayinclude information indicating that the UE 900 is in the dualconnectivity state, an identifier of the source SeNB for dualconnectivity, information indicating that the SeNB HO is ongoing, atarget SeNB identifier and source MeNB and SeNB flow information.

Thereafter, the target macro eNB 940 may transmit an HO Ack message tothe MeNB 910, at step S935.

Then, the MeNB 910 may transmit an HO trigger indicator (macro) to thetarget pico eNB 930, at step S940. The HO indicator may include theidentifier of the target macro eNB 940.

Further, the MeNB 810 may transmit the HO trigger indicator (macro) tothe source pico eNB 920, at step S945. The HO trigger indicator mayinclude the identifier of the target macro eNB 940.

Then, the target macro eNB 940 completes the HO process, at step S947.The target macro eNB 940 transmits a resource release (HO Ack) messageto the MeNB 918, at step S950.

The MeNB 910 transmits an HO success indicator (macro) to the targetpico eNB 930, at step S955. Further, the MeNB 910 transmits the HOsuccess indicator (macro) to the source pico eNB 920, at step S960.

Then, the target pico eNB 930 completes the HO process, at step S965,and transmits a resource release (HO Ack) message to the MeNB 910, atstep S970.

Then, the MeNB 910 transmits an HO success indicator (pico) to thesource pico eNB 920, at step S975. Further, the MeNB 910 transmits theHO success indicator (pico) to the target macro eNB 940, at step S980.

Thereafter, the UE 900 and the target pico eNB 930 transmit/receivedata, at step S985, and the UE 900 and the target macro eNB 940transmit/receive data, at step S990.

Next, a macro HO first start and a macro HO first end in the RAN splitwill be described.

In the above described embodiment of the present invention, after thepico eNB HO starts, the macro eNB, having determined the HO, transmitsan HO request, which includes information indicating that the UE is inthe dual connectivity state with a source macro eNB ID, informationindicating that the macro eNB HO is ongoing with the target macro eNBID, and information on all flows of the source pico eNB and the UE, tothe selected target pico eNB.

The target pico eNB performs an admission control and transmits an Ackto the source macro eNB.

The source macro eNB transmits a request for transmitting data to thesource pico eNB and transmits an HO command to the UE.

When the HO on the pico layer is ongoing and the HO on the macro layerhas been completed, the target macro eNB transmits the HO Ack to thesource macro eNB. The source macro eNB informs the source pico eNB andthe target pico eNB of the HO completion on the macro layer.

Data transmission may be performed by the target macro eNB together withthe source pico eNB. When the HO on the pico layer is completed, thetarget pico eNB transmits, to the source macro eNB, the HO Ack thatinforms the target macro eNB and the source pico eNB of the completionof the HO on the pico layer.

Data transmission may be performed by the target macro eNB togetherwith, the target pico eNB.

The above process will be described in detail with reference to FIG. 10.

FIG. 10 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a macro HO first end in a RAN split,according to an embodiment of the present invention.

A UE 1000 first detects triggering of an HO event on the macro link, atstep S1005. Then, the UE 1000 transmits a measurement report to an MeNB1010, at step S1010. The UE 1000 and a target macro eNB 1040 perform thesame process as that of the RAN split macro HO.

Thereafter, the UE 1000 detects triggering of an HO event on the picolink, at step S1020. Then, the UE 1000 transmits a measurement report tothe MeNB 1010, at step S1025. The MeNB 1010 transmits an HO requestmessage to a target pico eNB 1030, at step S1030. The HO request messagemay include information indicating that the MeNB 1010 HO is ongoing andidentification information of the target eNB is a MeNB ID, andinformation on SeNB flows.

Thereafter, the target SeNB 1030 may transmit an HO Ack message to theMeNB 1010, at step S1035.

Then, the MeNB 1010 may transmit an HO trigger request message to asource pico eNB 1020, at step S1040. The HO trigger request message mayinclude an identifier of the target pico eNB 1030.

Then, the source pico eNB 1020 transmits UE context information andforwards data to the target pico SeNB 1030, at step S1045.

In step S1050, the target macro eNB 1040 completes the HO process. Then,the target macro eNB 1040 transmits a resource release message (HOcompletion) to the MeNB 1010, at step S1055. As illustrated at stepS1060, pico flows have not yet been sent to the target pico eNB 1030.

The MeNB 1010 transmits an HO success indicator (macro) to the targetpico eNB 1030, at step S1065. The HO success indicator may includeidentification information on the target macro eNB 1040.

Further, the MeNB 1010 transmits the HO success indicator (macro) to thetarget pico eNB 1030, at step S1070. The HO success indicator mayinclude identification information on the target macro eNB 1040.

The target pico eNB 1030 completes the HO process. Then, the target picoeNB 1030 transmits a resource release message (HO completion) to theMeNB 1010, at step S1080. The MeNB 1010 transmits an HO successindicator (pico) to the source pico eNB 1020, at step S1085, andsubsequently transmits the HO success indicator (pico) to the targetmacro eNB 1040, at step S1090.

Thereafter, the UE 1000 transmits/receives data to/from the target picoeNB 1030, at step S1095, and transmits/receives data to/from the targetmacro eNB 1040, at step S1097.

Next, a macro HO first start and a pico HO first end in the RAN splitwill be described.

In the above described embodiment of the present invention, after thepico eNB HO starts, the macro eNB, having determined the HO, transmitsan HO request which includes information indicating that the UE is inthe dual connectivity state with a source macro eNB ID, informationindicating that the macro eNB HO is ongoing with the target macro eNBID, and information on all flows of the spume pico eNB and the UE, tothe selected target pico eNB.

The target pico eNB performs an admission control and transmits an Ackto the source macro eNB. The source macro eNB transmits a request fortransmitting data to the source pico eNB and transmits an HO command tothe UE. When the HO on the macro layer is ongoing and the HO on the picolayer has been completed, the target pico eNB transmits the HO Ack tothe source macro eNB.

The source macro eNB informs the source pico eNB and the target macroeNB of the HO completion on the pico layer. Data transmission may beperformed by the source macro eNB together with the target pico eNB.

When the HO on the macro layer is completed, the target macro eNBtransmits, to the source macro eNB, the HO Ack that informs the targetpico eNB of the completion of the HO on the macro layer. Datatransmission may be performed by the target macro eNB together with thetarget pico eNB.

The above embodiment will be described in detail with reference to FIG.11.

FIG. 11 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a pico HO first end in a RAN split,according to an embodiment of the present invention.

A UE 1100 first detects triggering of an HO event on the macro link, atstep S1105. Then, the UE 1100 transmits a measurement report to an MeNB1110, at step S1110. The UE 1100 and a target macro eNB 1140 perform thesame process as that of the RAN split macro HO.

Thereafter, the UE 1100 detects triggering of an HO event on the picolink, at step S1120. Then, the UE 1100 transmits a measurement report tothe MeNB 1110, at step S1125. The MeNB 1110 transmits an HO requestmessage to a target pico eNB 1130, at step S1130. The HO request messagemay include information indicating that the MeNB 1110 handover isongoing, identification information of the target eNB is an MeNB ID, andinformation on SeNB flows.

Thereafter, the target pico eNB 1130 may transmit an HO Ack message tothe MeNB 1110, at step S1135.

Then, the MeNB 1110 may transmit an HO trigger request message to asource pico eNB 1120, at step S1140. The HO trigger request message mayinclude an identifier of the target pico eNB 1130.

Then, the source pico eNB 1120 transmits UE 1100 context information andforwards data to the target pico eNB 1130, at step S1142.

At step S1145, the target pico eNB 1130 completes the HO process. Then,the target macro eNB 1140 transmits a resource release message (HOcompletion) to the MeNB 1110, at step S1150.

The MeNB 1110 transmits an HO success indicator (pico) to the targetmacro eNB 1140, at step S1155. The HO success indicator may includeidentification information on the target pico eNB 1130.

Further, the MeNB 1110 transmits an HO success indicator (pico) to thesource pico eNB 1120, at step S1160. The HO success indicator mayinclude identification information on the target pico eNB 1130.

At step S1165, the target macro eNB 1140 completes the HO process. Then,the target macro eNB 1140 transmits a resource release message (HOcompletion) to the MeNB a 1110, at step S1170. The MeNB 1110 transmitsan HO success indicator (macro) to the target pico eNB 1130, at stepS1175.

Thereafter, the UE 1100 transmits/receives data to/from the target picoeNB 1130, at step S1180, and transmits/receives data to/from the targetmacro eNB 1140, at step S1195.

Next, a pico HO First start and a pico HO first end in the CN split willbe described.

In the above described embodiment of the present invention, after themacro eNB HO starts, the macro eNB, having determined the HO, transmitsan HO request, which includes information indicating that the UE is inthe dual connectivity state with a source pico eNB ID, informationindicating that the pico eNB HO is ongoing with a target pico eNB ID,and information on all flows, to the selected target macro eNB.

The target macro eNB performs an admission control and transmits an Ackto the source macro eNB. The source macro eNB transmits the indicationindicating that the HO on the macro layer is ongoing with the targetmacro eNB ID to both the source and target pico eNBs, and transmits anHO command to the UE.

When the HO on the macro layer is ongoing and the HO on the pico layerhas been completed, the target pico eNB transmits the HO Ack to thesource macro eNB. The source macro eNB informs the source pico eNB andthe target macro eNB of the HO completion on the pico layer. Datatransmission may be performed by the source macro eNB together with thetarget pico eNB.

When the HO on the macro layer is completed, the target macro eNBtransmits, to the source macro eNB, the HO Ack that informs the targetpico eNB (active eNB on the pico layer) of the completion of the HO onthe macro layer. Data transmission may be performed by the target macroeNB together with the target pico eNB.

The above embodiment will be described in more detail with reference toFIG. 12.

FIG. 12 is a signaling diagram illustrating an HO process as it pertainsto a pico HO a first start and a pico HO first end in a CN split,according to an embodiment of the present invention.

A UE 1200 first detects triggering of an HO event on the pico link, atstep S1205. Then, the UE 1200 transmits a measurement report to an MeNB1210, at step S1210. The UE 1200 and a target pico eNB 1230 perform thesame process as that of the CN split pico HO.

Thereafter, the UE 1200 detects triggering of an HO event on the macrolink, at step S1220. Then, the UE 1200 transmits a measurement report tothe MeNB 1210, at step S1225. The MeNB 1210 transmits an HO requestmessage to a target macro MeNB 1240, at step S1230. The HO requestmessage may include information indicating that the UE 1200 is in thedual connectivity mode, information indicating that the UE 1200 is beingconnected to a source SeNB ID, information indicating that the SeNB HOis ongoing, identification information of the target eNB is an SeNB ID,and information on MeNB 1220 flows.

Thereafter, the target macro eNB 1240 may transmit an HO Ack message tothe MeNB 1210, at step S1235.

Then, the MeNB 1210 may transmit an HO trigger indicator (macro) to asource pico eNB 1220, at step S1240. The HO trigger request message mayinclude an identifier of the target macro eNB 1240.

Further, the MeNB 1210 may transmit the HO trigger indicator (macro) tothe target pico eNB 1230, at step S1245. The HO trigger request messagemay include an identifier of the target macro eNB 1240.

At step S1250, the target pico eNB 1230 completes the HO process. Then,the target pico eNB 1230 transmits a resource release message (HOcompletion) to the MeNB 1210, at step S1255.

The MeNB 1210 transmits an HO success indicator (pico) to the sourcepico eNB 1220, at step S1260. Further, the MeNB 1210 transmits the HOsuccess indicator (pico) to the target macro eNB 1240, at step S1265.

At step S1270, the target macro eNB 1240 completes the HO process. Then,the target macro eNB 1240 transmits a resource release message (HOcompletion) to the MeNB 1210, at step S1275. The MeNB 1210 transmits anHO success indicator (macro) to the target pico eNB 1230, at step S1280.The HO success indicator may include identification information on thetarget macro eNB 1240.

Thereafter, the UE 1200 transmits/receives data to/from the target picoeNB 1230, at step S1285, and transmits/receives data to/from the targetmacro eNB 1240, at step S1290.

Next, a pico HO first start and a macro HO first end in the CN splitwill be described.

In the above described embodiment of the present invention, after themacro eNB HO starts, the macro eNB, having determined the HO, transmitsan HO request, which includes information indicating that the UE is inthe dual connectivity state with a source pico eNB ID, informationindicating that the pico eNB HO is ongoing with a target pico eNB ID,and information on all flows, to the selected target macro eNB.

The target macro eNB performs an admission control and transmits an Ackto the source macro eNB. The source macro eNB transmits the indicationindicating that the HO on the macro layer is ongoing with the targetmacro eNB ID to both the source and target pico eNBs, and transmits anHO command to the UE.

When the HO on the pico layer is ongoing and the HO on the macro layerhas been completed, the target macro eNB transmits the HO Ack to thesource macro eNB. The source macro eNB informs the source pico eNB andthe target pico eNB of the HO completion on the macro layer. Datatransmission may be performed by the target macro eNB together with thesource pico eNB.

When the HO on the pico layer is completed, the target pico eNBtransmits, to the source macro eNB, the HO Ack that informs the targetmacro eNB and the some pico eNB of the completion of the HO on the picolayer. Data transmission may be performed by the target macro eNBtogether with the target pico eNB.

The above process will be described in detail with reference to FIG. 13.

FIG. 13 is a signaling diagram illustrating, an HO process as itpertains to a pico HO first start and a macro HO first end in a CNsplit, according to an embodiment of the present invention.

A UE 1300 first detects triggering of an HO event on the pico link, atstep S1305. Then, the UE 1300 transmits a measurement report to an MeNB1310, at step S1310. The UE 1300 and a target pico eNB 1330 perform thesame process as that of the CN split pico HO.

Thereafter, the UE 1300 detects triggering of an HO event on the macrolink, at step S1320. Then, the UE 1300 transmits a measurement report tothe MeNB 1310, at step S1325. The MeNB 1310 transmits an HO requestmessage to a target macro eNB 1340, at step S1330. The HO requestmessage may include information indicating that the UE is in the dualconnectivity mode and is being connected to a source SeNB ID, andinformation indicating that the SeNB HO is ongoing and identificationinformation of the target eNB is an SeNB ID, and information on the MeNB1310 flows.

Thereafter, the target MeNB 1340 may transmit an HO Ack message to theMeNB 1310, at step S1335.

Then, the MeNB 1310 may transmit an HO trigger indicator (macro) to asource pico eNB 1320, at step S1340. The HO trigger request message mayinclude an identifier of the target macro eNB 1340.

Further, the MeNB 1310 may transmit the HO trigger indicator (macro) tothe target pico eNB 1330, at step S1345. The HO trigger request messagemay include an identifier of the target macro eNB 1340.

At step S1050, the target macro eNB 1340 completes the HO process. Then,the target macro eNB 1340 transmits a resource release message (HOcompletion) to the MeNB 1310, at step S1355.

The MeNB 1310 transmits an HO success indicator (macro) to the sourcepico eNB 1320, at step S1360. The MeNB 1310 transmits an HO successindicator (macro) to the target macro eNB 1340, at step S1365.

At step S1370, the target pico eNB 1330 completes the HO process. Then,the target pico eNB 1330 transmits a resource release message (HOcompletion) to the MeNB 1310, at step S1375.

Then, the MeNB 1310 transmits an HO success indicator (pico) to thesource pico eNB 1320, at step S1380. Further, the MeNB 1310 transmitsthe HO success indicator (pico) to the target macro eNB 1340, at stepS1385.

Thereafter, the UE 1300 transmits/receives data to/from the target picoeNB 1330, at step S1390, and transmits/receives data to/from the targetmacro eNB 1340, at step S1395.

Next, a macro HO first start and a macro HO first end in the CN splitwill be described.

In the above described embodiment of the present invention, after thepico eNB HO starts, the macro eNB, having determined the HO, transmitsan HO request, which includes information indicating that the UE is inthe dual connectivity state with a source macro eNB ID, informationindicating that the macro eNB HO is ongoing with the target macro eNBID, and information on all flows of the source pico eNB and the UE, tothe selected target pico eNB.

The target pico eNB performs an admission control and transmits an Ackto the source macro eNB. The source macro eNB transmits a request fortransmitting data to the source pico eNB and transmits an HO command tothe UE.

When the HO on the pico layer is ongoing and the HO on the macro layerhas been completed, the target macro eNB transmits the HO Ack to thesource macro eNB. The source macro eNB informs the source pico eNB andthe target pico eNB of the HO in completion on the macro layer. Datatransmission may be performed by the target macro eNB together with thesource pico eNB.

When the HO on the pico layer is completed, the target pico eNBtransmits, to the source macro eNB, the HO Ack that informs the targetmacro eNB and the source pico eNB of the completion of the HO on thepico layer. Data transmission may be performed by the target macro eNBtogether with the target pico eNB.

The above process will be described in detail with reference to FIG. 14.

FIG. 14 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a macro HO first end in a CN split,according to an embodiment of the present invention.

A UE 1400 first detects triggering of an HO event on the macro link, atstep S1405. Then, the UE 1400 transmits a measurement report to an MeNB1410, at step S1410. The UE 1400 and a target macro eNB 1440 perform thesame process as that of the RAN split macro HO.

Thereafter, the UE 1400 detects triggering of an HO event on the picolink, at step S1420. Then, the UE 1400 transmits a measurement report tothe MeNB 1410, at step S1425. The MeNB 1410 transmits an HO requestmessage to a target pico eNB 1430, at step S1430. The HO request messagemay include information indicating that the MeNB 1410 HO is ongoing andidentification information of the target eNB is a MeNB ID, andinformation on SeNB flows.

Thereafter, the target pico eNB 1430 may transmit an HO Ack message tothe MeNB 1410, at step S1435.

Then, the MeNB 1410 may transmit an HO trigger request message to asource pico eNB 1420, at step S1440. The HO trigger request message mayinclude an identifier of the target pico eNB 1430.

Then, the source pico eNB 1420 transmits the UE 1400 context informationto the target pico eNB 1430, at step S1445.

At step S1450, the target macro eNB 1440 completes the HO process. Then,the target macro eNB 1440 transmits a resource release message (HOcompletion) to the MeNB 1410, at step S1455.

The MeNB 1410 transmits an HO success indicator (macro) to the targetpico eNB 1430, at step S1455. The HO success indicator may includeidentification information on the target macro eNB 1440.

Further, the MeNB 1410 transmits the HO success indicator (macro) to thesource pico eNB 1420, at step S1460. The HO success indicator mayinclude identification information on the target macro eNB 1440. Then,the source pico eNB 1420 may perform a conventional path switch process,at step S1465.

At step S1470, the target pico eNB 1430 completes the HO process. Then,the target pico eNB 1430 transmits a resource release message (HOcompletion) to the MeNB 1410, at step S1475. The MeNB 1410 transmits anHO success indicator (pico) to the source pico eNB 1420, at step S1450,and subsequently transmits the HO success indicator (pico) to the targetmacro eNB 1440, at step S1485.

Thereafter, the UE 1400 transmits/receives data to/from the target picoeNB 1430, at step S1490, and transmits/receives data to/from the targetmacro eNB 1440, at step S1495.

Next, a macro HO first start and a pico HO first end in the CN splitwill be described.

In the above described embodiment of the present invention, after thepico eNB starts, the macro eNB, having determined the HO, transmits anHO request, which includes information indicating that the UE is in thedual connectivity state with a source macro eNB ID, informationindicating that the macro eNB HO is ongoing with the target macro eNBID, and information on all flows of the source pico eNB and the UE, tothe selected target pico eNB.

The target pico eNB performs an admission control and transmits an Ackto the source macro eNB. The source macro eNB transmits a request fortransmitting data to the source pico eNB and transmits an HO command tothe UE. When the HO on the macro layer is ongoing and the HO on the picolayer has been completed, the target pico eNB transmits the HO Ack tothe source macro eNB.

The source macro eNB informs the source pico eNB and the target macroeNB of the HO completion on the pico layer. Data transmission may beperformed by the source macro eNB together with the target pico eNB.

When the HO on the macro layer is completed, the target macro eNBtransmits, to the source macro eNB, the HO Ack that informs the targetpico eNB of the completion of the HO on the macro layer. Datatransmission may be performed by the target macro eNB together with thetarget pico eNB.

The above process will be described in detail with reference to FIG. 15.

FIG. 15 is a signaling diagram illustrating an HO process as it pertainsto a macro HO first start and a pico HO first end in a CN split,according to an embodiment of the present invention.

A UE 1500 first detects triggering of an HO event on the macro link, atstep S1505. Then, the UE 1500 transmits a measurement report to an MeNB1510, at step S1510. The UE 1500 and a target macro eNB 1540 perform thesame process as that of the RAN split macro HO.

Thereafter, the UE 1500 detects triggering of an HO event on the picolink, at step S1520. Then, the UE 1500 transmits a measurement report tothe MeNB 1510, at step S1525. The MeNB 1510 transmits an HO requestmessage to a target pico eNB 1530, at step S1530. The HO request messagemay include information indicating that the MeNB 1510 HO is ongoing andidentification information of the target eNB is an MeNB ID, andinformation on SeNB flows.

Thereafter, the target pico eNB 1530 may transmit an HO Ack message tothe MeNB 1510, at step S1535.

Then, the MeNB 1510 may transmit an HO trigger request message to asource pico eNB 1520, at step S1540. The HO trigger request message mayinclude an identifier of the target SeNB.

Then, the source SeNB 1520 transmits the UE 1500 context information tothe target pico eNB 1530, at step S1545. Further, the source pico eNB1520 forwards data to the target pico eNB 1530, at step S1550.

Thereafter, a conventional path switch process may be performed betweenthe source pico eNB 1520 and the target pico eNB 1530, at step 1555.

At step S1560, the target pico eNB 1530 completes the HO process. Then,the target pico eNB 1530 transmits a resume release message (HOcompletion) to the MeNB 1510, at step S1565.

The MeNB 1510 transmits an HO success indicator (pico) to the targetmacro eNB 1540, at step S1570. The HO success indicator may includeidentification information on the target pico eNB 1530.

Further, the MeNB 1510 transmits the HO success indicator (pico) to thesource pico eNB 1520, at step S1575. The HO success indicator mayinclude identification information on the target pico eNB 1530.

At step S1580, the target macro eNB 1540 completes the HO process. Then,the target macro eNB 1540 transmits a resource release message (HOcompletion) to the MeNB 1510, at step S1585. The MeNB 1510 transmits anHO success indicator (macro) to the is target pico eNB 1530, at stepS1590. The HO success indicator may include an identifier of the targetmacro eNB 1540.

Thereafter, the UE 1500 transmits/receives data to/from the target picoeNB 1530, at step S1595, and transmits/receives data to/from the targetmacro eNB 1540, at step S1597.

Common characteristics of the dual link hand over described in FIGS.8-15 are summarized as follows.

HO request

-   -   Information content    -   Indicate UE DC mode together with related eNB ID    -   Flows of related eNB (except for flows thereof)        -   RAN split    -   Layer on which HO is ongoing    -   Target BID (base station ID) of layer on which HO is ongoing    -   Use    -   Macro HO        -   Source macro→target macro    -   Pico HO        -   Source macro→target pico

HO Trigger IND

-   -   Information content    -   Layer of HO    -   Target eNB ID

UE ID

-   -   Use    -   Macro HO        -   Source macro→source pico        -   Source macro→target pico            -   When macro HO is triggered while pico HO is ongoing

HO Trigger REQ

-   -   Information content

UE ID

-   -   Use    -   Pico HO        -   Source macro→source pico            -   Trigger pico HO                -   In different way from conventional way

HO Success IND

-   -   Information content    -   Layer on which HO is completed    -   UE ID    -   Use    -   Source macro→source pico    -   Source macro→target pico        -   When pico HO is ongoing while macro HO ends    -   Source macro→target macro    -   When macro HO is ongoing while pico HO ends

Meanwhile, according to another embodiment of the present invention, thepico link HO process in the CN split scenario may be taken into accountas follows.

When the macro eNB determines the HO and then the HO of the pico eNB inthe dual connectivity state is performed, the macro eNB transmits an HOcommand for triggering an HO process for the pico layer to the sourcepico eNB.

When receiving the trigger from the macro eNB, the source pico eNBtransmits an HO request including information indicating that the UE isin the dual connectivity state with a related macro eNB ID to the targetpico eNB.

The target pico eNB performs an admission control and transmits an Ackto the source pico eNB. The source pico eNB informs the macro eNB of aresult of the admission control by the target pico eNB. When result ofthe admission control is positive, the macro eNB transmits an HO commandto the UE. The source pico eNB and the target pico eNB continuouslyperform the conventional HO execution and completion processes. When thetarget pico eNB transmits the HO Ack to the source pico eNB, the sourcepico eNB notifies the completion of the HO on the pico layer to themacro eNB. Data transmission is performed by the target pico eNBtogether with the macro eNB providing the UE through dual connectivity.

The above process will be described in detail with reference to FIG. 16.

FIG. 16 is a signaling diagram illustrating an HO process, according toanother embodiment of the present invention.

A UE 1600 first detects triggering of an HO event on the pico link, atstep S1605. Then, the UE 1600 transmits a measurement report to an MeNB1610, at step S1610.

The MeNB 1610 determines whether to perform the HO, at step S1615. Inthe determination, the MeNB 1610 transmits an HO trigger request message(pico) to a source pico eNB 1620, at step S1620. The HO trigger requestmessage may include identification information of the target pico eNB1630.

The source pico eNB 1620 transmits an HO request message to the targetpico eNB 1630, at step S1620. The HO request message may includeinformation indicating that the UE 1600 is in the dual connectivitymode, identification information of the MeNB 1610 connected to the UE1600 through dual connectivity, and flow information related to thesource pico eNB 1620.

Then, the target pico eNB 1630 transmits an HO Ack message to the sourcepico eNB 1620, at step S1630, and the source pico eNB 1620 transmits anHO trigger response message to the MeNB 1610, at step S1635.

The MeNB 1610 transmits an HO command message to the UE 1600, at stepS1640.

Thereafter, at step S1642, a conventional HO execution process isperformed between the source pico eNB 1620 and the target pico eNB 1630.Further, the target pico eNB 1630 transmits an HO completion message tothe HE 1600, at step S1645.

Thereafter, the UE 1600 and the target pico eNB 1630 transmit/receivedata to/from each other.

At step S1655, the conventional path switch process is performed betweenthe source pico eNB 1620 and the target pico eNB 1630. Then, the targetpico eNB 1630 transmits a resource release message (HO Ack) to thesource pico eNB 1620, at step S1660. Then, the source pico eNB 1620transmits an HO success indicator (pico) to the MeNB 1610, at stepS1665.

At step S1670, dual connectivity is formed between the MeNB 1610 and thetarget pico eNB 1630.

Main characteristics of the embodiment of FIG. 16 are summarized astallows.

HO Preparation Phase

-   -   HO trigger REQ    -   Source macro→source pico    -   Trigger conventional HO between source and target pico eNBs    -   HO request        -   Source pico→target pico        -   Indicate that UE is in DC state with source macro (ID)    -   HO trigger RSP    -   Source pico→source macro    -   Source macro eNB transmits HO command to UE

HO Execution Phase

-   -   Apply conventional HO process

HO Completion Phase

-   -   Apply conventional HO process    -   HO success IND    -   Source pico→source macro    -   Source macro can perform DC together with target pico

HO Request

-   -   Indicate UE DC mode (together with source macro ID)    -   Target pico readily accepts DC mode

HO Trigger REQ

-   -   Target pico ID    -   Trigger conventional HO together with target pico    -   UE ID

HO Trigger RSP

-   -   Success/Failure    -   Trigger HO command to UE

HO Success IND

-   -   Pico layer    -   Source macro performs DC with target pico    -   UE ID

FIG. 17 is a block diagram illustrating an internal structure of an eNBwhich can be applied to various eNBs, according to an embodiment of thepresent invention.

The internal structure of the eNB illustrated in FIG. 17 may be aninternal structure of any eNB such as a macro eNB or a pico eNB, andchanges may be made such that the characteristics described in each ofthe embodiments are executed by functions performed by a controlleraccording to various embodiments of the present invention.

For example, a transceiver 1710 may transmit/receive a signal to/front aUE or another eNB. The eNB may transmit/receive a signal such as acontrol signal or data to/from the UE through a radio channel. Further,the eNB may transmit/receive a measurement report message transmittedfrom the UE through the transceiver 1710. One eNB may be connected toanother eNB or nodes (for example, SGW air MME) in a core networkthrough a wired interface to transmit/receive signals.

A controller 1720 may control signal flows between blocks of the UE tosupport the HO of the UE in a wireless communication system in which theeNB supports dual connectivity, according to an embodiment of thepresent invention.

For example, when the eNB is a master eNB and a pico eNB is handed over,the controller 1720 may transmit an HO request message to a target slaveeNB when a slave eNB HO for the UE is determined. Further, when a HO Ackmessage is received from the target slave eNB, the controller 1720 maytransmit an HO trigger request message including identificationinformation of the target slave eNB to a source slave eNB. Further, whena resource release message is received from the target slave eNB, thecontroller 1720 may transmit an HO success indicator to the source slaveeNB.

In contrast, when the eNB is a master eNB and a macro eNB is handedover, the controller 1720 may transmit an HO request message to astarget master eNB when a master eNB HO for the UE is determined. Whenreceiving a HO Ack message from the target master eNB, the controller1720 may transmit an HO trigger indicator to a source slave eNB.Further, when a resource release message is received from the targetmacro eNB, the controller 1720 may transmit an HO success indicator tothe source slave eNB.

As described above, the block diagram illustrating the internalstructure of the eNB of FIG. 17 may be construed so that the respectiveeNBs perform various functions in the various embodiments describedabove.

In all the above embodiments, data and state forwarding may include oneor more downlink PDCP (Packet Data Convergence Protocol) state reports,authorized downlink PDUs, and unauthorized uplink PDUs. According to anembodiment of the present invention, instead of forwarding the data andstate to the target eNB, the salve eNB transmits the data and state tothe master eNB and sequentially forwards the data and state to thetarget eNB. According to another embodiment of the present invention,the master eNB may buffer data transmitted to the salve eNB in a RANsplit model, hi this case, the slave eNB does not transmit unauthorizeddownlink PDUs to the master eNB.

According to an embodiment, when the HO of the master eNB is triggered,additional data may be pushed to the salve eNB for data transmission tothe UE. In uplink, the UE may make a request for additional resources totransmit the additional data to the salve eNB. When the UE receives anHO command for the master eNB, the UE transmits a Buffer Status Report(BSR) to the slave eNB by including pending data to be transmitted tothe master eNB.

In all the above embodiments, the master eNB was referred to as themaster eNB and the piers eNB was referred to as the slave eNB. However,a predetermined type eNB may operate as the master or slave eNB.

According to the above described embodiments of the present invention,an efficient UE HO process can be performed in a wireless communicationsystem supporting dual connectivity.

While the present invention has been shown and described with referenceto various embodiments thereof, it should be understood by those skilledin the art that many variations and modifications of the method andapparatus described herein will still fall within the spirit and scopeof the present invention as defined in the appended claims and theirequivalents.

What is claimed is:
 1. A method performed by a master base station in awireless communication system, the method comprising: transmitting, to atarget secondary base station, a request message for a handover of auser equipment (UE) from a source secondary base station to the targetsecondary base station; receiving, from the target secondary basestation, an acknowledgment message in response to the request message;and transmitting, to the source secondary base station, a messageincluding an identifier for data forwarding for a radio access network(RAN) split bearer, wherein data is forwarded from the source secondarybase station to the target secondary base station based on theidentifier, and wherein a handover procedure with a core network is nottriggered for the RAN split bearer that is configured for the masterbase station and the source secondary base station.
 2. The method ofclaim 1, wherein the identifier for data forwarding includes anidentifier of the target secondary base station, and wherein the requestmessage includes information indicating that the UE is in a dualconnectivity mode.
 3. The method of claim 1, further comprising:transmitting, to the UE, a trigger message for the UE to apply a newconfiguration based on configuration information included in the requestmessage.
 4. A method performed by a source secondary base station in awireless communication system, the method comprising: receiving, from amaster base station, a message including an identifier for dataforwarding for a radio access network (RAN) split bearer in case that ahandover of a user equipment (UE) from the source secondary base stationto a target secondary base station is requested; forwarding data for theUE to the target secondary base station based on the identifier; andreleasing resources associated with the UE, wherein a handover procedurewith a core network is not triggered for the RAN split bearer that isconfigured for the master base station and the source secondary basestation.
 5. The method of claim 4, wherein the identifier for dataforwarding includes an identifier of the target secondary base station,and wherein information indicating that the UE is in a dual connectivitymode is transmitted from the master base station to the target secondarybase station.
 6. The method of claim 4, further comprising: transferringa UE context to the target secondary base station based on theidentifier for data forwarding.
 7. A method performed by a targetsecondary base station in a wireless communication system, the methodcomprising: receiving, from a master base station, a request message fora handover of a user equipment (UE) from a source secondary base stationto the target secondary base station; transmitting, to the master basestation, an acknowledgment message in response to the request message;and receiving, from the source secondary base station, data for the UEthat is transmitted based on a data forwarding identifier for dataforwarding for a radio access network (RAN) split bearer, wherein thedata forwarding identifier is transmitted from the master base stationto the source secondary base station, and wherein a handover procedurewith a core network is not triggered for the RAN split bearer that isconfigured for the master base station and the source secondary basestation.
 8. The method of claim 7, wherein the data forwardingidentifier includes an identifier of the target secondary base station,and wherein the request message includes information indicating that theUE is in a dual connectivity mode.
 9. The method of claim 7, furthercomprising: synchronizing with the UE.
 10. A master base station in awireless communication system, the master base station comprising: atransceiver; and a controller configured to: transmit, via thetransceiver to a target secondary base station, a request message for ahandover the of a user equipment (UE) from a source secondary basestation to the target secondary base station, receive, via thetransceiver from the target secondary base station, an acknowledgmentmessage in response to the request message, and transmit, via thetransceiver to the source secondary base station, a message including anidentifier for data forwarding for a radio access network (RAN) splitbearer, wherein data is forwarded from the source secondary base stationto the target secondary base station based on the identifier, andwherein a handover procedure with a core network is not triggered forthe RAN split bearer that is configured for the master base station andthe source secondary base station.
 11. The master base station of claim10, wherein the identifier for data forwarding includes an identifier ofthe target secondary base station, and wherein the request messageincludes information indicating that the UE is in a dual connectivitymode.
 12. The master base station of claim 10, wherein the controllertransmits, to the UE, a trigger message for the UE to apply a newconfiguration based on configuration information included in the requestmessage.
 13. A source secondary base station in a wireless communicationsystem, the source secondary base station comprising: a transceiver; anda controller configured to: receive, via the transceiver from a masterbase station, a message including an identifier for data forwarding fora radio access network (RAN) split bearer in case that a handover of auser equipment (UE) from the source secondary base station to a targetsecondary base station is requested, forward data for the UE to thetarget secondary base station based on the identifier, and releaseresources associated with the UE, wherein a handover procedure with acore network is not triggered for the RAN split bearer that isconfigured for the master base station and the source secondary basestation.
 14. The source secondary base station of claim 13, wherein theidentifier for data forwarding includes an identifier of the targetsecondary base station, and wherein information indicating that the UEis in a dual connectivity mode is transmitted from the master basestation to the target secondary base station.
 15. The source secondarybase station of claim 13, wherein the controller is further configuredto transfer a UE context to the target secondary base station based onthe identifier for data forwarding.
 16. A target secondary base stationin a wireless communication system, the target secondary base stationcomprising: a transceiver; and a controller configured to: receive, viathe transceiver from a master base station, a request message for ahandover the of a user equipment (UE) from a source secondary basestation to the target secondary base station, transmit, via thetransceiver to the master base station, an acknowledgment message inresponse to the request message, and receive, via the transceiver fromthe source secondary base station, data for the UE that is transmittedbased on a data forwarding identifier for data forwarding for a radioaccess network (RAN) split bearer, wherein the data forwardingidentifier is transmitted from the master base station to the sourcesecondary base station, and wherein a handover procedure with a corenetwork is not triggered for the RAN split bearer that is configured forthe master base station and the source secondary base station.
 17. Thetarget secondary base station of claim 16, wherein the identifier fordata forwarding includes an identifier of the target secondary basestation, and wherein the request message includes information indicatingthat the UE is in a dual connectivity mode.
 18. The target secondarybase station of claim 16, wherein the controller is further configuredto synchronize with the UE.